Over the past century the demand for energy has grown exponentially. With the growing demand for energy, many different energy sources have been explored and developed. One of the primary sources for energy has been and continues to be the combustion of hydrocarbons. However, the combustion of hydrocarbons usually results in incomplete combustion and non-combustibles that contribute to smog and other pollutants in varying amounts.
As a result of the pollutants created by the combustion of hydrocarbons, the desire for cleaner energy sources has increased in more recent years. With the increased interest in cleaner energy sources, fuel cells have become more popular and more sophisticated. Research and development on fuel cells has continued to the point that many speculate that fuel cells will soon compete with the gas turbine for generating large amounts of electricity for cities, the internal combustion engine for powering automobiles, and batteries that run a variety of small and large electronics.
Fuel cells conduct an electrochemical energy conversion of a fuel and an oxidizer into electricity and heat. Fuel cells are similar to batteries, but they can be “recharged” while providing power.
Fuel cells provide a direct current (DC) voltage that may be used to power motors, lights, or any number of electrical appliances. There are several different types of fuel cells, each using a different chemistry. Fuel cells are usually classified by the type of electrolyte used. The fuel cell types are generally categorized into one of five groups: proton exchange membrane (PEM) fuel cells, alkaline fuel cells (AFC), phosphoric-acid fuel cells (PAFC), solid oxide fuel cells (SOFC), and molten carbonate fuel cells (MCFC).
Solid Oxide Fuel Cells
The SOFCs are currently believed to be a very promising fuel cell technology, and allow the use of a variety of fuels (e.g., hydrogen, hydrocarbons, alcohols, etc.). Referring to FIG. 1, a SOFC will typically include four basic elements: an anode (20), a cathode (22), an electrolyte (24), and bipolar plates (26) arranged on each side of the electrolyte (24) in contact with the respective anode (20) or cathode (22).
The bipolar plate (26) in contact with the anode (20) is the negative post of the fuel cell and conducts electrons that are freed from the fuel such that the electrons can be used in an external circuit (21). The bipolar plate (26) includes channels (28) etched therein to disperse the fuel as evenly as possible over the surface of the anode (20), and remove any fuel products from the fuel anode reaction (e.g., water, carbon dioxide, etc.).
The bipolar plate (26) in contact with the cathode (22) is the positive post of the fuel cell, and has channels (30) etched therein to evenly distribute oxygen (usually air) to the surface of the cathode (22), and provide for the removal of oxygen depleted air. The cathode (22) also conducts the electrons back from the external circuit to the catalyst, where they combine with molecular oxygen to form oxygen ions.
The electrolyte (24) is a solid oxide membrane (24). The membrane (24) is typically a high temperature ceramic material that conducts only oxygen ions. This membrane (24) also prevents the passage of electrons.
The anode (20) is typically a ceramic/metal mixture (cermet) (e.g., yttria stabilized zirconia/nickel, samaria doped ceria/nickel, etc.). The anode (20) is usually porous so as to maximize the three-phase boundary. The anode (20) facilitates the oxidation of the fuel.
The cathode (22) is typically a composite mixture of the electrocatalyst and oxygen ion conductor (e.g., lanthanum strontium maganate/yttria stabilized zirconia, samarium strontium cobaltite/samaria doped ceria, etc.). The cathode (22) is usually porous so as to maximize the three-phase boundary. The cathode (22) facilitates the reduction of the oxidant.
In a working fuel cell, the solid oxide membrane (24) is sandwiched between the anode (20) and the cathode (22). The operation of the fuel cell can be described generally as follows. The fuel (e.g., hydrocarbon, H2, carbon monoxide, etc.) enters the fuel cell on the anode (20) side. When the fuel comes into contact with the catalyst on the anode (26), ions and electrons are formed, where for the case of an H2 molecule two H+ ions and two electrons (e−) are formed. The electrons are conducted through the anode (20) to the bipolar plate (26), where they make their way through the external circuit (21) that may be providing power to do useful work (such as turning a motor or lighting a bulb (23)) and return to the cathode side of the fuel cell.
Meanwhile, on the cathode (22) side of the fuel cell, molecular oxygen (O2) is present in air and is flowing through the catalyst (26). As O2 is forced through the catalyst (26), it forms two oxygen ions, each having a strong negative charge. These oxygen ions pass through the solid oxide electrolyte and interact with the fuel on the anode. In the case of H2 as the fuel, the oxygen ions combine to form a water molecule and two electrons for the external circuit.
The fuel cell reaction just described produces only about 0.7 volts at a useful current, therefore, to raise the voltage to a higher level, many separate fuel cells are often combined to form a fuel cell stack.
Solid oxide fuel cells typically operate at fairly high temperatures (above approximately 800° C.), which allow them to have high reaction kinetics, and use a variety of fuels depending on the anode composition. Lower temperature operation is desired for applications that require rapid startup, where inexpensive containment structures, and temperature management is of concern.
The reaction rate for the dissociation and ionization of molecular O2, normally provided by ambient air, and the transport of these ions to the electrolyte limits the power of the cell especially when operating temperatures are reduced (<600° C.). This phenomena (insufficient availability of oxygen ions present at the cathode/electrolyte interface) in part causes the cathode overpotential and it limits the performance of fuel cells even with the use of advanced materials such as doped cobaltites, manganites, and ferrites, when operated at low temperatures. Two methods can be used to limit this effect. One method is to provide pure oxygen to the cathode (22) to ensure a sufficient supply of O2 is always present to maximize the reaction rate and thus the electricity produced. The second method is to run the cell at higher operating temperatures to increase the oxygen dissociation kinetics and ion transport. However, a pure supply of O2 adds to the expense of the fuel cell operation and may even render the operation of the fuel cell inefficient and/or uneconomical if a pure supply of O2 is necessary to facilitate the reaction. Also, operating at higher temperatures may limit the packaging options and add to the cost of the system.